1. Field of the Invention
The present invention relates generally to a recording-medium reproducing apparatus and a recording-medium reproducing method. More particularly, the invention relates to a recording-medium reproducing apparatus for reproducing recorded information from a plurality of types of disc-like recording mediums with substrates of different thicknesses. The invention is also concerned with a recording-medium reproducing method used in the above type of apparatus.
2. Description of the Related Art
As recording mediums from which information is reproduced by using light, in addition to compact discs (CD) (trade name) which are currently widely used, new recording mediums, such as digital video discs (DVD) having a much larger capacity than CDs, which are capable of recording long periods of video pictures in a digital form, are being considered. Digital information is optically read from the above type of recording medium in the following manner. Laser light is applied to the recording medium, and the reflected light from the medium is detected. Then, the level of reflected light is converted into binary data.
FIG. 8 illustrates an example of the construction of an optical pick-up device used for a CD. The optical pick-up device has a laser diode (LD) 1 emitting laser light having a wavelength of 780 nm. A beam splitter 2, formed of a transparent plane parallel plate, reflects the laser light emitted from the LD 1 toward an objective 3. The beam splitter 2 also provides astigmatism for the reflected light (convergent light) that is returned from a CD 10 via the objective 3 and transmits the light toward a photodiode 4.
The objective 3 converges the laser light and directs it onto an information recording layer 12 of the CD 10 on which miniscule pits are arranged. The objective 3 is also adapted to converge the light reflected from the information recording layer 12 of the CD 10 so as to apply the light to the photodiode 4 via the beam splitter 2. The objective 3 having a larger numerical aperture (NA) can converge light at a greater angle into a smaller area. In this example, the objective 3 having a NA of 0.45 is used.
The photodiode 4 is adapted to detect the laser light which has been applied to the CD 10 from the LD 1 and returned from the CD 10. Astigmatism is generated in the laser light reflected by the information recording layer 12 when the light passes through the beam splitter 2 before being incident on the photodiode 4. Focus servo control is performed by utilizing this astigmatism.
The CD 10 used as a recording medium is constructed in such a manner that the information recording layer 12 is formed on a transparent substrate 11 having a thickness t of 1.2 mm, and a protective film 13 is further deposited on the information recording layer 12. Laser light emitted from the LD 1 penetrates the substrate 11 and is further applied to the information recording layer 12. The information recording layer 12 has small pits corresponding to recorded information. Diffraction occurs to the laser light incident on the pits so as to lower the intensity of the returned light (the light reflected by the CD 10 and applied to the photodiode 4). On the other hand, laser light applied to the information recording layer 12 on which the pits are not formed is directly reflected to increase the intensity of the returned light. Such two types of returned light are detected by the photodiode 4, and the high and low levels of intensity of the light are converted into "1" and "0". As a consequence, digital data recorded as pits on the CD 10 can be read.
In this manner, recorded information can be read by applying laser light to predetermined positions in the recording medium and by further detecting the light returned from the medium.
In addition to the CD 10 discussed above, a DVD 20 constructed as shown in FIG. 9 has been proposed. In contrast to the CD 10 having information on only one side thereof, it is possible to record information on both sides of the DVD 20. More specifically, the DVD 20 is constructed of the following two blocks. An information recording layer 22 is formed on a transparent substrate 21, and a protective film 23 is further disposed on the information recording layer 22. Moreover, an information recording layer 32 is formed on a transparent substrate 31, and a protective film 33 is further deposited on the recording layer 32. The resulting two blocks are bonded in such a manner that the two protective films 23 and 33 opposedly face each other. Accordingly, the structure of the DVD 20 constructed as described above is symmetrical, as illustrated in FIG. 9.
Since the DVD 20 can record information with higher density than the CD 10, the pit length and the pit gap of the DVD 20 are made smaller than those of the CD 10. Thus, an objective 43 having a larger NA (0.6) is used in a DVD-designed optical pick-up device, because light is applied to the more minute pits in the DVD 20. The use of the objective 43 having a larger NA (0.6) makes it possible to converge laser light into a smaller area so that the smaller pits can be read.
Because the above-described discs (such as the CD 10 and the DVD 20) are generally mass-produced, some discs (for example, polycarbonate-made inexpensive discs) are disadvantageously skewed by as much as 0.5 to 1 degree. Thus, the following problem is encountered when data reproduction is performed from such a skewed disc. Data reading is disadvantageously carried out by applying laser light and detecting the returned light while a disc is skewed from the optical axis, thereby causing wavefront aberration. This makes an image-forming spot on the disc asymmetrical, increasing interference between codes and further generating large distortion in an output signal, which may hamper correct data reading. The major aberration generated due to this disc skewing is third-order aberration, which is proportional to the cube of the NA, the angle of disc skewing, and the thickness of a substrate. Accordingly, third-order aberration is more responsive to disc skewing in accordance with a larger NA.
Thus, since reading is performed by an objective having a large NA (0.6) in the DVD 20, the third-order aberration is likely to increase by the use of a substrate having a thickness similar to that of the CD 10. In order to reduce the third-order aberration, the thicknesses of the substrates 21 and 31 are made smaller than the thickness of the substrate 11 of the CD 10. More specifically, the thickness of the substrates 21 and 31 of the DVD 20 are 0.6 mm, while the thickness of the substrate 11 of the CD 10 is 1.2 mm.
Further, the recording density of the DVD 20 is larger than that of the CD 10. Consequently, a LD 41 for use in a DVD-designed optical pick-up device is the one that emits laser light having a wavelength (635 nm) shorter than that emitted from the CD-designed LD 1. Additionally, the objective 43 having a larger NA is used.
According to the above description, information can be read from the CD 10 or the DVD 20 only by the use of an optical system (an optical pick-up device) designed specifically for the CD 10 or the DVD 20. It is difficult, however, to accurately read information from both the CD 10 and the DVD 20 with the use of only a single optical system.
For example, if an optical pick-up device designed specifically for a DVD is applied to the CD 10, as shown in FIG. 10, the following problems are presented. That is, if the optical pick-up device used for the DVD 20, which is optimized to read data stored in the DVD 20, reads data from the CD 10, an adverse influence of fourth-order aberration (proportional to the fourth power of the NA and the thickness of the substrate) is produced due to a difference in the thickness between the substrate 11 of the CD 10 and the substrates 21 and 31 of the DVD 20 and a difference in the NA between the objectives 3 and 43. This deteriorates the characteristics of a focus error signal used in the focus servo control (a correct focus error signal has symmetrical positive and negative peaks formed in an "S" shape in the vicinity of the focal position), as illustrated in FIG. 11, making it difficult to detect the focal position, thus failing to obtain a correct data signal (RF signal). Hence, it is difficult to perform the focus servo control, which may even hamper data detection.
To solve this problem, the present inventor previously proposed the following method, such as the one disclosed in Japanese Patent Application No. 7-302415. In this method, a small-sized photodetector is used in a CD to prevent the detection of the peripheral portion of returned light during data reading, which peripheral portion is particularly vulnerable to an adverse influence produced by spherical aberration. Thus, the adverse influence of spherical aberration can be inhibited, thereby making it possible to read data from a CD with a DVD-designed optical system. In this method, a photodiode is used, such as one sized to receive light returned from a DVD or a CD corresponding only to an NA of approximately 0.3 or lower.
However, when data is reproduced from a DVD according to the above-described method, this small-sized photodetector fails to obtain a sufficient level of output signal, unlike a known type of photodetector used for a DVD. This deteriorates the focus error signal and the data output signal (RF signal) when DVD reading is performed.